50道真题中的第29题，看了答案还是不太明白，那位高手能给解释一下啊！谢谢啦！
29. a frequency report of the variable Jobcode in the work.Actors data set is listed below
Jobcode frequency percent cumulative
frequency                      cumulative
percent
Actor I   2                        33.33   2        33.33
Actor II  2                        33.33   4        66.67
Actor III 2                        33.33   6        100.00
                                        frequency missing=1

The following sas program is submitted:
data work.joblevels;
     set work.actors;
     if jobcode in ('Actor I', 'Actor II') then
        joblevel='Beginner';
     if jobcode='Actor III' then
        joblevel='Advanced';
     else joblevel='Unknown';
run;

Which of the following represents the possible value for the variable joblevel in the work.joblevels data set?
a. Advanced and Unknown only
b. Beginner and Advanced only
c. Beginner, Advanced, and Unknown
d.''(missing character value)

答案选a